The present invention relates to data storage systems and, more particularly, to a transfer curve tester for testing magnetic recording heads used in data storage systems.
Many data storage systems use magnetic or magneto-optical recording heads for writing information to and reading information from a magnetic medium. For example, disc drives of the xe2x80x9cWinchesterxe2x80x9d type have one or more rigid discs, which are coated with a magnetizable medium for storing digital information in a plurality of circular, concentric data tracks. The discs are mounted on a spindle motor, which causes the discs to spin and the surfaces of the discs to pass under respective head suspension assemblies. Head suspension assemblies carry transducers which write information to and read information from the disc surface. An actuator mechanism moves the head suspension assemblies from track-to-track across the surfaces of the discs under control of electronic circuitry. xe2x80x9cFloppy-typexe2x80x9d disc drives use flexible discs, which also have circular, concentric data tracks. For a tape drive, the information is stored along linear tracks on the tape surface.
In these applications, several different types of transducers have been used that rely on magnetic properties for writing to and/or reading from the magnetic medium. For an inductive-type transducer, the direction of current through the transducer is controlled during a write operation to encode magnetic flux reversals on the surface of the medium within the selected data track. When retrieving data from the medium, the inductive transducer is positioned over the data track to sense the flux reversals stored in the data track and generate a read signal based on those flux reversals. In a magneto-resistive type of transducing head, the flux reversals cause a change in the resistance of the head, which is sensed by a detector circuit. Typically, a reference current is passed through the magneto-resistive head and the change in resistance is sensed by measuring changes in the voltage across the head. Other types of detecting circuits can also be used.
In order to understand the basic physics of a magnetic transducing head during development and manufacturing, it is common to test the response of the head to an applied magnetic field. For example, one series of tests is known as xe2x80x9cTransfer Curve Testingxe2x80x9d. To generate a transfer curve for a particular transducing head, the head is placed in a magnetic field (steady state or time varying) and the output signal from the transducing head is measured. The transfer curve is simply a plot of the output signal versus the applied magnetic field, where the field is varied from some negative value to some positive value, which is usually the same magnitude as the negative value. For a magneto-resistive type of head, the output signal consists of a steady state voltage, which is a function of the bias current applied to the head, the bulk resistance of the head and the applied magnetic field. Typical characteristics that can be measured from the transfer curve data include read signal amplitude at maximum field, noise with zero field, noise with applied field, linearity over some range of field, and symmetry. Symmetry is a comparison of the read signal amplitude with a maximum positive field and the read signal amplitude with a maximum negative field.
The rapidly changing technology in magnetic recording heads has created a wide range of operating requirements for the heads as well as a wide range of head performances. For example, reference bias current requirements for a transfer curve tester can vary from tens of micro-Amperes to many tens of milli-Amperes, and the transfer curve tester may require tens of volts to drive the reference current. For magneto-resistive types of heads, the amplitudes of output voltages that must be measured can range from tens of micro-volts to tens of milivolts, while the resistance of the head can range from tens of Ohms to hundreds of Ohms. Also, the steady-state voltage output due to the reference bias current is typically hundreds of millivolts, but can be as large as tens of volts with special devices.
These wide ranges of operating requirements and head performances set very challenging requirements for the measurement electronics. For example, in order to measure the noise of a 50 Ohm head, the noise introduced by the transfer curve tester should be less than 1 nV/Hz. Bias currents of 10 micro-Amperes require a current source with an accuracy of better than 100 nAmps, and the input bias currents drawn by the measurement electronics should be similar to prevent measurement errors. All of these requirements, when coupled with a potentially large DC bias voltage, present a difficult design challenge for the measurement electronics in the transfer curve tester. Typically one or more of these requirements is substantially compromised.
Thus, a transfer curve tester having improved measurement electronics and an accurate current source is desired.
One aspect of the present invention is directed to a head testing apparatus for testing an output of a magnetic data recording head. The apparatus includes first and second tester input terminals for coupling to the head, first and second input stages and an AC-coupled differential amplifier. The first input stage has a first buffer input, which is coupled to the first tester input terminal, a first buffer output and a plurality of parallel-connected buffer amplifiers coupled between the first buffer input and the first buffer output. The second input stage has a second buffer input, which is coupled to the second tester input terminal, a second buffer output and a plurality of parallel-connected buffer amplifiers coupled between the second buffer input and the second buffer output. The AC-coupled differential amplifier has first and second amplifier inputs coupled to the first and second buffer outputs through first and second capacitors, respectively, and has a measurement output.
Another aspect of the present invention is directed to a method of testing a magnetic recording head. The method includes applying a bias current and a magnetic field to the head and coupling first and second outputs of the head to first and second input stages, respectively. Each input stage has a plurality of parallel-connected amplifiers. An output of each of the first and second input stages is coupled to a respective input of a differential amplifier through a respective AC coupling capacitance. A response of the head due to the applied bias current and the applied magnetic field is then measured based on an output of the differential amplifier.
Yet another aspect of the present invention relates to a balanced bi-directional current source, which includes first and second current output terminals, a current control circuit, first and second amplifiers and first and second sense impedances. The current control circuit has a reference input, a feedback input, and a control output, which is based on the reference input and the feedback input. The first amplifier has a first amplifier input, which is coupled to the control output, and a first amplified output. The second amplifier has a second amplifier input, which is coupled to the control output, and a second amplified output, which is inverted relative to the first amplified output. The first sense impedance is coupled in series between the first amplified output and the first current output terminal and has a feedback output which is coupled to the feedback input. The second sense impedance is matched with the first impedance and is coupled in series between the second amplified output and the second current output terminal. yet another aspect of the present invention relates to a voltage measurement circuit, which measures a voltage across first and second input terminals with an AC coupled differential amplifier, which blocks a DC component of the voltage, and presents a low input bias current to the first and second input terminals while isolating the first and second input terminals from charge stored in capacitances within the AC coupled differential amplifier.